The invention relates to a high-pressure sodium discharge lamp provided with an elongate discharge vessel enclosing a discharge space and having a longitudinal axis 1 and a diameter D and having a ceramic wall, which is closed at each end by a lead-through member, which is connected on the one hand to a relevant current conductor and on the other hand to a relevant electrode, the lamp further being provided with an outer envelope, which is closed by a lamp cap, and a starting circuit having at least a glow starter being arranged within said outer envelope. The term "ceramic wall" is to be understood herein to mean a wall constituted by translucent crystalline metal oxide, which may be monocrystalline (for example sapphire) or polycrystalline. Known polycrystalline metal oxides in this connection are aluminium oxide and yttrium-aluminium-garnet. In polycrystalline form, the material is sintered to gas-tightness.
A lamp of the kind-mentioned in the opening paragraph is known from U.S. Pat. No. 4,117,370. The known lamp is an efficient light source, which is frequently used. The lamp is provided with an ovoidal outer envelope, which is coated in practice on its inner side with a light-scattering layer. The glow starter is mounted in line with the discharge vessel so as to be enclosed between an end thereof and the mount of the lamp. It is desirable to mount the glow starter near the discharge vessel within the outer envelope because of the comparatively simple production, in contrast, for example, with mounting in the lamp cap.
The known lamp is suitable from a viewpoint of efficiency as a substitute for, for example, high-pressure mercury discharge lamps. If, however, an optimal substitution should be obtained, the substitute lamp should be arranged in an existing luminaire optically in substantially the same position as the original lamp. This means that especially the position of the discharge vessel with respect to the luminaire is of importance. As a measure for determining the position of the discharge vessel, the distance between the lamp cap bottom and the center of the discharge vessel is used. This distance is generally designated as light center length.
The discharge vessel of high-pressure mercury and high-pressure metal halide discharge lamps are considerably shorter than discharge vessels of high-pressure sodium discharge lamps suitable as substitutes. By coating the outer envelope with a light-scattering layer, it is achieved to a considerable extent that the positioning of the discharge vessel with respect to the luminaire in which the lamp is used is of minor importance.
However, a light-scattering layer limits the choice of the shape of the envelope to an ovoidal or similar shape, in order to achieve that the temperature of the light-scattering layer during operation of the lamp remains acceptable.
Although the use of a coated outer envelope has the advantage that positioning of the discharge vessel is not particularly critical, the optical behaviour of such a lamp will still be of lower quality than that of a similar lamp having a clear outer envelope and an optimally positoned discharge vessel. Moreover, the use of a light-scattering layer will always lead to loss of efficiency.